The invention relates to a device for the electrophoretic coating of the internal surface of electrically conductive hollow bodies which have an opening, in particular of packaging cans.
In the immersion method known from the European Patent Application 0 118 756, the hollow bodies are led in one continuous operation through an electrophoretic immersion coating bath in such a way that they are quickly and completely flooded with immersion bath fluid so that in the immersion bath they can be electrophoretically coated with a wet film. After sufficient coating time, the hollow bodies are lifted out of the immersion bath again and the immersion bath fluid located in them is poured out. After this, the hollow bodies coated in this way are fed mutually spaced from one another to a drying oven and dried therein, after which they can be printed on or labeled.
In this immersion method, the hollow bodies can only be electrophoretically coated on the outside and inside in a uniform way and they must be led into the electrophoretic coating bath in such a way that they are immersed with the opening pointing upward in order to be flooded and coated quickly and completely, after which they are lifted out of the immersion bath and have to be turned with the opening pointing downward so that the immersion bath fluid can run off. The result of this is that both the internal surface and the external surface must basically be coated with the same coating agent, which is not always required. Furthermore, a complex mechanism for immersing, lifting out and turning the hollow bodies has to be provided.
In the method described in the European Patent Application 0 431 711 a complex mechanism for immersing and lifting out the hollow bodies in an immersion bath is in fact not necessary since the hollow bodies are pushed with the opening pointing downward by means of an electrically conductive grille, the surface coating agent being rinsed, in jets which rise from below, in or over the hollow bodies through the free intermediate spaces between the grille bars or grating bars and at the same time covering the entire internal and external surface of the hollow bodies with an uninterrupted coating. In this process, the rising jet forms an uninterrupted conductive connection from an anode or cathode in a collection basin to the surface, forming the cathode or anode, of the hollow bodies. However, even with this method both the internal surface and the external surface are coated with the same electrophoretic surface coating agent.
In contrast, in the German Offenlegungsschrift 32 20 310 a method and a device for the simultaneous electrophoretic coating of the internal and external surfaces of a hollow body are described, with which method and device it is possible to coat the internal and external surfaces differently and with different surface coating agents. For this purpose, the hollow body is enclosed, with the opening turned downward, in a housing and a nozzle device is introduced into its interior, two separate through-passages being produced between the housing and container on the one hand and the container and nozzle device on the other, which through-passages are each provided with an inlet and an outlet so that they can be quickly filled with the electrophoretic coating material and the latter can be quickly discharged from them again. In this process, the coating material floods over the internal and external surfaces of the container and, owing to an electrical potential which is applied between the housing and container on the one hand and container and nozzle device on the other, is deposited on the surfaces of the hollow body. Two separate through-passages, each with a separate inlet and outlet, are provided for the coating material so that the internal and external surfaces of the container can be simultaneously coated with different coating materials and even with different thicknesses of coating.
A disadvantage with this device is that it is of complicated design since it is principally designed for the simultaneous coating of the external and internal surface of the hollow body. Furthermore, the movable nozzle must be introduced as far as the base of the hollow body so that the movements to be performed for each hollow body give rise to a considerable waste of time and it is not possible to achieve coating quickly, in particular of packaging cans.